The present invention relates to glass bubbles for use as fillers in a plastic resin and a method for producing the bubbles and, in particular, to such glass bubbles which are adaptable for fillers in a substrate of an electrical printed circuit board.
Glass bubbles or hollow glass spheres have been used as fillers in plastic resin articles so as to reinforce the articles. U.S. Pat. No. 5,292,690 discloses glass bubbles which have an improved compressive strength. The glass bubbles also have the reduced alkali amount leached from the glass so as to improve the adhesive strength. In order to reduce the alkali amount leached from the glass, contents of alkali elements in the composition of the glass bubbles are reduced. Thus, U.S. Pat. No. 5,292,690 discloses a composition for the glass bubbles which consists essentially of, by weight, SiO.sub.2 40-59%, R.sub.2 O 2-17%, R.sub.2 O being at least one selected from Na.sub.2 O, K.sub.2 O, and Li.sub.2 O, B.sub.2 O.sub.3 1-25%, RO 5-25%, RO being at least One selected from CaO, MgO, BaO, ZnO and SrO, RO.sub.2 6-40%, RO.sub.2 being at least one selected from TiO.sub.2 and ZrO.sub.2, Al.sub.2 O.sub.3 0-13%, P.sub.2 O5 0-3%, and SO.sub.3 0.1-1.0%. Bubble-forming powders of the composition are reheated to form glass bubbles. The glass bubbles have diameters of 1-130 .mu.m and wall thickness of 0.5-10 .mu.m and consist essentially of, by weight, SiO.sub.2 42-58%, R.sub.2 O 3-16%, R.sub.2 O being at least one selected from Na.sub.2 O, K.sub.2 O, and Li.sub.2 O, B.sub.2 O.sub.3 3.5-23%, RO 6-22%, RO being at least one selected from CaO, MgO, BaO, ZnO and SrO, RO.sub.2 7-26%, RO.sub.2 being at least one selected from TiO.sub.2 and ZrO.sub.2, Al.sub.2 O.sub.3 0-13%, P.sub.2 O5 0-2%, and SO.sub.3 0.2-0.8%.
Since glass bubbles are hollow members, they are light in weight and low in dielectric constant in comparison with own dielectric constant of the glass forming the bubbles. Accordingly, it has recently been proposed to use glass bubbles for fillers in plastic substrates of electrical printed circuit boards.
However, glass bubbles known in the art are not suitable for fillers in the printed circuit boards because alkali amount leached therefrom is not sufficiently suppressed but badly effects the electrical insulation resistance of plastic substrates of the electrical printed circuit boards. Although the glass bubbles disclosed in U.S. Pat. No. 5,292,690 are reduced in alkali contents and therefore in the alkali amount leached, the alkali amount leached is required to be further reduced for use in the electrical printed circuit boards.
Reduction of alkali contents in the glass for forming the bubbles results in reduction of SO.sub.3 contents in the glass and therefore reduces SO.sub.2 gas generated from the SO.sub.3 contents so that production of bubbles is made difficult.